1. Field of the Invention
This invention relates generally to a method and apparatus for detecting the position of a tape when recording or reproducing signals thereon.
2. Description of the Prior Art
In a conventional tape recorder, a timer roller is in rolling engagement with the tape so as to be rotated in response to movement of the tape between the supply and take-up reels on which the tape is wound. The length of the tape that is transported is directly measured on the basis of the number of revolutions of the timer roller, and the tape position can be determined from the transported tape length, while the amount of recording time that remains can be determined from such transported tape length and the speed at which the tape is being transported or advanced. It is also known to frequency divide the signal pulses provided as the output of the timer roller, whereupon the frequency divided output can be supplied to a counter which acts as a time indicator. In the foregoing case, the precision with which the tape position can be measured is determined by the accuracy with which the diameter of the timer roller is maintained during manufacture and also upon the slippage between the timer roller and the driven tape. Thus, it is necessary to pay strict attention to the precision with which the timer roller is machined, and to the uniformity of its material and bearing structure, and the like, whereby the cost of production of the timer roller is increased. Further, the tape is prone to be damaged due to its frictional contact with the timer roller for rotating the latter.
It is also known to provide a control signal, such as, a CTL signal, which is pre-recorded on the tape and thereafter reproduced as the tape is transported during a recording or reproducing operation for use as a clock pulse signal by a timer counter which indicates the tape position on the basis of the counted control signals that have been reproduced. However, this known method cannot be applied when recording on a blank tape.
It is also known to detect the tape position by initially squaring the numbers of pulses generated in response to rotations of the reels so as to determine the total amount of the tape on the reels, and then to calculate the absolute time of the tape position by employing the determined amount of the tape and the length of the tape that has been transported for ascertaining the amount of the tape that remains to be recorded or reproduced.
In accordance with still another known method for determining a tape position, the numbers of revolutions of the supply and take-up reels, respectively, are detected during the transportation of the tape through a relatively short distance, and a voltage is obtained proportional to the ratio of such numbers of revolutions, whereupon such voltage is convereted into a tape position indication. However, since the ratio of the numbers of revolutions of the supply and take-up reels is determined only during the transport of the tape through a relatively short distance, a monotonically increasing function is not obtained by reason of the quantization error in the rotation or non-circularity of a reel or reels. Therefore, it is difficult to ensure proper recognition of slight angular displacement of the reels during transport of the tape. Moreover, it is not possible with the existing system to achieve high precision in determining the tape position, for example, as when it is necessary to reproduce a given tape position for cueing or the like.